88 EXPERIMENTS, &c. 



occasioned by the action of its muscles. No 

 object, therefore, ought to appear separate and 

 distinct from others, if the answer were true 

 which I am combating ; but, on the contrary, 

 all those to which we successively direct our 

 eyes during the limits of the duration of an 

 impression upon the retina, should seem 

 crowded into one place; and, consequently, 

 none of them should be perceived with any 

 tolerable accuracy. Such are the conclusions 

 from the truth of this answer. I need scarcely 

 mention, that they are contradicted by expe- 

 rience. 



There is another form of the same fact, to 

 which, it may be thought, an explanation taken 

 from the duration of impressions on the retina 

 will better apply ; I mean the appearance of a 

 fiery circle, when any red-hot body is moved 

 quickly round. But it seems, to me, that such 

 an explanation cannot even here be admitted. 

 For, if the circle depended upon the cause I 

 have mentioned, it could only be observed as 

 long as the impressions upon the retina were 

 also disposed in the form of a circle. Were this 

 broken upon, which it must be by every move- 

 ment of the eye, the appearance suggested by 

 the last impression would no longer be so ar- 

 ranged, with respect to the appearance suggested 

 by the present impression, as to lie with it in the 



